[0001] The invention relates to a method for determining, by means of a radar apparatus,
the height of a low-elevation target, the radar apparatus being provided with a transmitting
and receiving unit to which are connected a transmitting and receiving antenna and
a signal processing unit, whereby:
- the target is illuminated by electromagnetic radiation emitted by the transmitting
unit and the transmitting antenna;
- the transmitted signal is reflected directly via the target and indirectly via the
target and the earth surface in the direction of the receiving antenna;
- by means of the receiving unit, complex sum signals Σ and complex elevation difference
signals Δ are derived from signals received by means of the receiving antenna;
- by means of the signal processing unit, an algorithm is carried out for deriving the
height of the target from the complex sum and elevation difference signals.
[0002] The invention furthermore relates to an apparatus for determining the height of a
low-elevation target, comprising a radar apparatus provided with a transmitting unit
to which a transmitting antenna is connected, a receiving antenna to which a receiving
unit is connected, whereby the target is illuminated by electromagnetic radiation
emitted by the transmitting unit and the transmitting antenna and the transmitted
signal is reflected directly by the target and by the target via the earth surface
in the direction of the receiving antenna and whereby, by means of the receiving unit,
complex sum signals Σ and at least complex elevation difference signals Δ, which are
representative for the target, can be generated, a signal processor connected to the
receiving unit, provided with an algorithm for determining the height of the target
h
t, and means connected to the signal processor for directing the transmitting antenna
and the receiving antenna at an aiming point.
[0003] The height of a target can be determined by means of a monopulse radar apparatus.
The monopulse principle is described in "Introduction to Radar Systems" by M.I. Skolnik,
second edition, pages 160-190. A problem encountered in the determination of the height
of a target which is flying at a low altitude above the sea surface and, to a lesser
extent, also the earth surface, is described on pages 172-176. This problem is due
to the phenomenon that the radar apparatus not only receives echo signals directly
from the target, but also target echoes reflected by the sea surface. Without the
occurrence of this multipath effect, the height of the target can be easily derived
from the values of the target range R and the elevation error voltage E(ϑ), delivered
by the monopulse radar apparatus. The multipath effect causes the elevation error
voltage to assume a complex value which renders an accurate determination of the target
height impossible.
[0004] The occurrence of said multipath effect can only be avoided by selecting a radar
antenna beamwidth which is narrow to such an extent that only echo signals directly
from the target are received, thereby excluding unwanted target echoes reflected by
the sea surface. This method has the drawback, however, that with such a narrow beam,
a relatively long time might be required to acquire the target. The patent US-A 4,743,907
provides an elegant solution to obviate this drawback. By fully integrating two monopulse
radar apparatuses, the first radar apparatus having a relatively large wavelength
and a consequent wide beam, the second radar apparatus having a relatively small wavelength
and a consequent narrow beam, it is possible to achieve an optimal performance in
both the acquisition and tracking phases. Such a system, however, entails considerable
cost.
[0005] A method for accurately determining target height, notwithstanding the occurrence
of multipath, is described in the patent US-A 4,769,031. According to the method disclosed
in this patent, the radar antenna is not directed at the actual target, but at a point
in space exactly between the actual target and its image. By subsequently measuring
with at least two different radar wavelengths, which bear a predetermined relation,
a system of equations can be solved, from which several possible target heights can
be derived. Thus, by successively comparing possible target heights derived on the
basis of this method, the target height can be determined.
[0006] An alternative method for accurately determining the target height is described in
patent EP-B 0.087.355. According to this method the antenna orientation is constantly
varied. Using measuring values obtained at various antenna orientations, it will again
be possible to solve systems of equations, from which the target height can be derived.
[0007] Both methods are cumbersome and have the drawback that the monopulse radar apparatus
is directed at an aiming point which does not coincide with the target. For a monopulse
radar apparatus, this misalignment is an evident suboptimal condition which, however,
in light of the state of the art is required in order to obtain a solvable system
of equations.
[0008] According to the invention the height of a target can be determined on the basis
of a method which is characterised in that the transmitting and receiving antenna
are substantially directed at the target.
[0009] Furthermore, the method described is relatively simple and is characterised in that
the height of the target h
t is determined by solving the following equation:
[0010] The apparatus according to the invention is characterised in that the target and
the aiming point at least substantially coincide.
[0011] The invention will be further explained with reference to the following figures,
of which:
- Fig. 1
- presents a diagram of a possible embodiment of a monopulse radar apparatus according
to the invention;
- Fig. 2
- presents a possible elevation error voltage curve;
- Fig. 3
- illustrates the multipath effect.
[0012] For accurately determining the height of a target flying at a low altitude above
the sea or earth surface, a monopulse radar apparatus is used. In this situation,
sum signals and elevation difference signals delivered by the radar apparatus assume
complex values, which are to be further processed by a signal processor connected
to the radar apparatus.
[0013] Fig. 1 presents a diagram of a possible embodiment of a monopulse radar apparatus
according to the invention. In order not to unnecessarily complicate the description,
only the monopulse behaviour in elevation is included in the analysis.
[0014] Two antenna elements 1, 2, one placed on top of the other, are connected to a conventional
coupler 3, forming a sum channel 4 and a difference channel 5. A transmitter 7, which
is controlled from a frequency and timing unit 8 and which transmits pulsed signals,
is connected to the sum channel 4 via a TR-switch 6. Signals received via the sum
channel are fed via TR-switch 6 to a mixer stage 9, which receives an LO signal from
the frequency and timing unit 8. The resulting intermediate-frequency signal is amplified
in intermediate-frequency amplifier 10 and converted into a digital, complex sum signal
Σ by A/D converter and Hilbert filter 11. Signals received via the difference channel
5 are fed to a mixer stage 12, which also receives an LO channel from the frequency
and timing unit 8. The resulting intermediate-frequency difference signal is amplified
in intermediate-frequency amplifier 13 and converted into a digital, complex difference
signal Δ by A/D converter and Hilbert filter 14. Both signals are fed to a signal
processor 15, by means of which an elevation error voltage S = Δ/Σ is determined.
The A/D converter and Hilbert filters 11, 14 may be replaced by phase-sensitive quadrature
detectors plus A/D converters, but the embodiment described here is more satisfactory
with regard to gain and offset stability.
[0015] Assuming there is a single stationary target in the beam, the vectors Σ and Δ will
remain identical for each transmitted pulse and will have a fixed angle in the complex
plane. For a single moving target in the beam, both vectors will rotate at the doppler
frequency, but will retain the same fixed angle. It is common practice to apply a
phase-alignment point in one of both channels, such that both vectors can be aligned.
For a single target the elevation error voltage will then be real. The real elevation
error voltage curve E(ϑ) is represented in Fig. 2. It is noted that the elevation
error voltage, notwithstanding its name, is dimensionless. Insofar as this is relevant,
is is possible to linearize E(ϑ) in signal processor 15, at least for small values
of ϑ; this may be effected by means of a linearizing table. In that case E(ϑ) = K.ϑ
applies for small angles G. Furthermore, signal processor 15 may generate an AGC control
signal for adjusting the gain of both intermediate-frequency amplifiers 10, 13 such
that the amplitude of the target echo signal in the sum signal Σ is kept substantially
constant. This results in less stringent requirements as regards the phase tracking
of the sum channel and the difference channel; it allows the use of A/D converters
with a limited dynamic range, and limits the size of the linearizing table.
[0016] In general, signal processor 15 will perform several other functions which are of
minor relevance to the invention described here. Thus, a conventional time-gate function
will be implemented in signal processor 15. Also, a form of MTI or MTD doppler processing
will be applied to the values delivered by the A/D converters and Hilbert filters
11, 14. In addition, signal processor 15 will generate control signals for directing
the transmitting antenna and the receiving antenna.
[0017] In the event of a single target in the beam, an echo of which is received directly
and as a mirror image via the sea surface, as represented in Fig. 3, S proves to assume
a complex value. In Fig. 3 h
a represents the height of the antenna above the sea surface, h
t represents the height of the target above the sea surface, R represents the range
from target to radar antenna and ϑ
o represents the antenna elevation angle. For a moving target, S becomes a function
of the target range R, the target height h
t, the radar transmitter wavelength λ and of several system constants. Our objective
is to find an equation which incorporates these values and from which h
t can be derived. This equation will then constitute the basis for the claimed method
and apparatus.
[0018] For forming this equation we define:
- ha
- height of the antenna above the sea surface.
- ht
- height of the target above the sea surface.
- R
- range from target to radar antenna.
- ϑo
- antenna elevation angle.
- ϑt
- elevation error angle of the target.
- ϑm
- elevation error angle of the mirror image.
- ρ
- reflection coefficient of the sea surface.
- Ψ
- perturbation phase for the reflection on the sea surface.
- φ
- phase difference between the reflections of target and mirror image.
- GΣ(ϑ)
- antenna diagram of the elevation sum channel.
- GΔ(ϑ
- ) antenna diagram of the elevation difference channel.
- E(ϑ)
- elevation error voltage curve.
[0019] The following approximations can now be derived:
Furthermore the following applies:
Subsequently we can define:
The last equation is based on the odd symmetry of E(ϑ).
[0020] We define a corrected reflection coefficient G, thereby considering that in case
of a monopulse antenna directed at the target, the reflection of the mirror image
is additionally attenuated by the antenna diagram:
Subsequently it follows from (3), (7), (8), (9) and (10) that:
During normal target tracking, the antenna is directed at the target, consequently
A = 0:
The real part of S can be defined as follows:
For the argument of S, the following applies:
hence:
A combination of (13) and (15) yields the desired equation:
A closer examination reveals that h
t and Ψ are the only unknowns in (16) in addition to several system parameters and
measuring values.
[0021] For a smooth sea surface we may assume that:
Together with (16) this yields:
Assuming that E(ϑ) is linear for small values of ϑ, this is a quadratic equation in
h
t. If E(ϑ) is not linear, the equation can be solved, for example following the Newton
method. From a series of target height estimates, thus obtained in time, the best
target height estimate is derived by a conventional filtering process with a time
constant and a provision for eliminating extremely deviating estimates, a method well
known in the art.
[0022] A second solution shall be selected, if the smooth sea surface condition does not
apply, consequently if
[0023] We can then eliminate the unknown Ψ by measuring at two different wavelengths. The
following can be derived from (16):
For minor wavelength differences the following approximation can be used:
With (3), (9) and (19) this yields:
This equation can be solved by means of one of the methods described under (18).
[0024] The values of h
t thus obtained can be used for directing the transmitting antenna and the receiving
antenna at the target. In this way an elevation error angle is realised which may
show an improvement by an order of magnitude as compared against a monopulse radar
apparatus where the imaginary part of the elevation error voltage is ignored. In addition,
this method is comparatively insusceptible to perturbations, particularly A = 0 proves
to be a non-stringent condition.
1. Method for determining, by means of a radar apparatus, the height of a low-elevation
target, the radar apparatus being provided with a transmitting and receiving unit
to which are connected a transmitting and receiving antenna and a signal processing
unit, whereby:
- the target is illuminated by electromagnetic radiation emitted by the transmitting
unit and the transmitting antenna;
- the transmitted signal is reflected directly via the target and indirectly via the
target and the earth surface in the direction of the receiving antenna;
- by means of the receiving unit, complex sum signals Σ and complex elevation difference
signals Δ are derived from signals received by means of the receiving antenna;
- by means of the signal processing unit, an algorithm is carried out for deriving
the height of the target from the complex sum and elevation difference signals,
characterised in that the transmitting and receiving antenna are substantially directed
at the target.
2. Method as claimed in claim 1, whereby on the basis of the algorithm, the target range
R and the complex elevation error voltage S = Δ/Σ are derived from the complex signals
Σ and Δ, characterised in that the height h
t of the target is determined by solving the equation:
3. Method as claimed in claim 2, whereby, for the transmitting antenna, the receiving
antenna and the receiving unit, a real error voltage curve E(ϑ) is known for a target
with an elevation error angle ϑ, the transmitted signal has a wavelength λ, the receiving
antenna is positioned at a height h
a above the earth surface and makes an elevation angle ϑ
o with the earth surface, characterised in that the equation
is solved.
4. Method as claimed in claim 2, whereby for the transmitting antenna, the receiving
antenna and the receiving unit, a real elevation error curve E(ϑ) is known for a target
with an elevation angle ϑ, the transmitting antenna successively transmits at least
signals of wavelengths λ₁ and λ₂, in which λ₁ ≠ λ₂, the receiving antenna is positioned
at a height ha above the earth surface and makes an elevation angle ϑ
o with the earth surface and whereby the values
are determined, characterised in that for determining h
t the equation
is solved.
5. Apparatus for determining the height of a low-elevation target, comprising a radar
apparatus provided with a transmitting unit to which a transmitting antenna is connected,
a receiving antenna to which a receiving unit is connected, whereby the target is
illuminated by electromagnetic radiation emitted by the transmitting unit and the
transmitting antenna and the transmitted signal is reflected directly by the target
and by the target via the earth surface in the direction of the receiving antenna
and whereby, by means of the receiving unit, complex sum signals Σ and at least complex
elevation difference signals Δ, which are representative for the target, can be generated,
a signal processor connected to the receiving unit, provided with an algorithm for
determining the height of the target ht, and means connected to the signal processor for directing the transmitting antenna
and the receiving antenna at an aiming point, characterised in that the target and
the aiming point at least substantially coincide.
6. Apparatus as claimed in claim 5, whereby from the signals Σ and A the complex elevation
error voltage S = Δ/Σ, and the target range R are derived, characterised in that the
algorithm solves the following equation:
7. Apparatus as claimed in claim 6, whereby for the transmitting antenna, the receiving
antenna and the receiving unit, a real error voltage curve E(ϑ) is known for a target
with an elevation error angle ϑ, the transmittted signal has a wavelength λ, the receiving
antenna is positioned at a height h
a above the earth surface and makes an elevation angle ϑ
o with the earth surface, characterised in that
is determined.
8. Apparatus as claimed in claim 6, whereby for the radar antenna, the receiving antenna
and the receiving unit, a real error voltage curve E(ϑ) is known for a target with
an elevation angle ϑ, the transmitting antenna being provided with means for successively
generating transmitter signals of wavelengths λ₁ and λ₂ in which λ₁ ≠ λ₂ and whereby
the values
are determined, the receiving antenna is positioned at a height h
a above the earth surface and makes an elevation angle ϑ
o with the earth surface, characterised in that the algorithm derives h
t from the following equation:
9. Signal processor provided with an algorithm as described in one of the claims 5 through
8.